Such apparatuses for determining and monitoring fill level in a container are often used in measuring devices in automation- and process control-technology. For example, measuring devices under the names Micropilot, Prosonic and Levelflex are produced and sold by Endress+Hauser. These measuring devices function according to the travel-time measurement method, and serve for determining and/or monitoring a fill level of a medium in a container. In the travel time measuring method, for example, microwaves, or radar waves, are transmitted via an antenna into the process space or are guided on a surface waveguide extending into the fill substance, and the reflected echo waves are received back by the antenna, or measurement transmitter, following a distance-dependent, travel time of the signals. From the time difference between the transmission of the high frequency signals and receipt of the reflected, echo signals, the distance from the measuring device to the surface of the medium can be ascertained. Apparatuses and methods for determining fill level via the travel time of measurement signals utilize the physical principle, according to which travel distance is equal to the product of travel time and propagation velocity. Taking into consideration the geometry of the interior of the container, the fill level of the medium can be ascertained as a relative or absolute variable. The travel-time method can be subdivided into essentially two basic methods: Thus, one method involves measuring the time difference, which a pulse-modulated, high frequency, measuring signal requires for the traveled path; another widely distributed ascertainment method involves the determining of the sweep-frequency difference of the transmitted, continuous, high frequency signal relative to the reflected, received, high frequency signal (FMCW—Frequency-Modulated Continuous Wave). In the following, no limitation to a particular ascertainment method is intended, but, instead, a travel time method, in general, is applied.
A general problem in the case of all travel time measuring methods utilizing high frequency measuring signals in the GHz range is that, for evaluating the high frequency, total measuring signal, composed of transmitted measuring signals and reflected, echo signals, high frequency building blocks must be used, designed for such high frequency ranges. A solution for this problem involves sequential sampling, which produces from a plurality of such high frequency, sampled, measuring signals, by sampling the recorded, sampled measuring signals in defined, periodic intervals, a time-expanded, intermediate frequency signal. This additional processing of the high frequency measuring signals is performed, because there are no appropriately cost-favorable, data processing units, e.g. DSPs (digital signal processors), which can reliably process high frequency measuring signals.
In the present state of the art, there are a number of approaches for producing a low frequency, intermediate frequency signal from a plurality of sampled measurement signals by a method of sequential sampling, or sampling method. Time expansion, or time delay, by means of sequential sampling is based on making sure that the time difference between two sampling points is constant to a high degree. For a long time, two methods have been known that are suitable for realizing this requirement, these being the mixer principle and the ramp principle, with the ramp principle being quantized and working only approximately continuously.
Thus, a first approach for producing a time-expanded, intermediate frequency signal is the mixer principle, in the case of which two oscillators produce two oscillations of slightly different frequencies. By the slight difference of the frequencies of the two oscillations, a linear phase shift is obtained which rises with each measuring period, this corresponding to a linearly increasing time delay.
The mixer principle is described, for example, in DE 31 07 444 A1 in the context of a high resolution, pulse radar method. A generator produces first microwave pulses and radiates them via an antenna at a predetermined transmission repetition frequency in the direction of the surface of the fill substance. Another generator produces reference microwave pulses, which are equal to the first microwave pulses, except that their transmission repetition frequency is slightly different. The echo signal and the reference signal are mixed, for example, by a frequency converter, or mixer, whereby an intermediate frequency signal is obtained. The intermediate frequency signal has the same behavior as the echo signal except that it is time expanded in comparison therewith by a translation factor equal to a quotient of the transmission repetition frequency and the frequency difference between the repetition frequency, or pulse repetition frequency, of the first microwave pulses and the repetition frequency, or sampling frequency, of the reference microwave pulses. In the case of a transmission repetition frequency of some megahertz, a frequency difference of a few hertz and a microwave frequency of some gigahertz, the frequency of the intermediate frequency signal lies well below 100 kilohertz. The advantage of the transformation to the intermediate frequency is that relatively slow, and, therefore, cost-favorable, electronic components can be used for signal registering and/or signal evaluation. Referenced also in this connection is the German Gebrauchsmuster DE 29815069 U1, which describes the known transformation technology in the case of a TDR, fill-level, measuring device. This sampling circuit has two oscillators, of which at least one is embodied to have a variable frequency, with one oscillator controlling the transmission generator and the other oscillator controlling the sampling pulse generator. A frequency mixer forms from the two frequencies the difference used for setting, or controlling, the time expansion factor, which is as constant as possible, to a desired value.
Another apparatus for time transformation of high-frequency measuring signals is disclosed in WO 2004/013584 A1. In this case, the frequency shift between the pulse repetition frequency and the sampling frequency is produced by two phase-coupled control loops having differently adjusted forwards and backwards dividers, the so-called phase-locked loop.
In the case of the second approach—the ramp principle —, the time difference equidistant from sampling point to sampling point is produced with the assistance of an RC member. The RC member is pre-charged by a step-shaped or linearly rising, ramp voltage, receives, thus, a certain offset, and is, then, charged and discharged in the rhythm of the transmission repetition frequency. With increasing ramp voltage, the voltage offset in the RC members increases, whereby the reaching of the switching threshold is delayed as a function of the level of the ramp voltage. The time expansion factor, or the transformation factor, is, in the ramp principle, definitively dependent on the time constant of an RC member. The dependence of the RC member on temperature acts fully on the scaling of the time transformation. In order, approximately, to remove this problem, it is necessary to compensate the temperature-related changes via a control loop. But, even with this measure, the scaling of the time transformation changes as a function of temperature in such a manner that the requirements for highly accurate measurements are not fulfilled. This ramp principle is disclosed in U.S. Pat. No. 5,563,605. The construction of the phase delay circuit disclosed therein has the disadvantage that considerable phase jitter, or instabilities, occur in the clocking frequencies. A further, general disadvantage facing the ramp principle is the considerable effort that must be expended to produce the ramp voltage.